The present invention relates generally to stereophonic sound reproduction systems, and in particular to such a system wherein the stereo signals are processed to produce illusion of expanded stage width to a listener with acceptable sound quality for input stereophonic signals with different degrees of correlation between right and left channels.
U.S. Pat. application No. 772,149 filed Feb. 25, 1977 now U.S. Pat. No. 4,118,599 and assigned to the same assignee of the present invention discloses stage-expanded systems for reproduction of stereophonic signals. The system disclosed in FIG. 21 of the application comprises a binaural localization network and a crosstalk cancellation network connected in tandem therewith to drive loudspeakers with binaurally correlated localized, crosstalk-free signals. The binaural localization network comprises a first adder having a first input connected to receive a right-channel stereo signal, a second adder having a first input in receipt of a left-channel stereo signal, a first subtractor which provides algebraic subtration between the right-channel input signal and the output signal from the second adder to provide a subtraction output which is coupled through a first transfer circuit to a second input of the first adder so that the direct right-channel input signal and the translated right-channel signal are combined together in reverse phase and the direct right-channel signal and and the left-channel output signal are combined together in phase. This transfer circuit has a transfer function B.sub.i /A.sub.i where A.sub.i is the transfer function of an acoustic paths between hypothetical loudspeakers and the nearside ears of a listener and B.sub.i is the transfer function acoustic crosstalk paths between the hypothetical speakers and the far-side ears of the listener. In the same fashion, algebraic subtraction is effected in a second subtractor between the left-channel input signal and the right-channel output signal from the first adder to supply its output through a second transfer circuit having the same transfer function as the first transfer circuit to the second input of the second adder so that the direct left-channel signal and the translated left-channel signal are combined together in reverse phase and the direct left-channel signal and the right-channel output signal are combined together in phase. The crosstalk cancellation circuit of the disclosed system is receptive of the right- and left-channel localized output signals from the localization network to develop a pair of output signals for application to the loudspeakers without producing the effect of acoustic crosstalk which might be perceptible by the listener if the localized output signals from the localization network were separately supplied directly to the loudspeakers.
Mathematical analysis of the prior art system has revealed that when the input signals have a high degree of correlation, that is, those signals derived from a sound source located at or near the center of the stage width, the system ensures good sound quality reproduction. However, when the input signals have a lesser degree of correlation, that is, those signals derived from different sound sources located separately at the extreme ends of the stage width, the sound quality was found unacceptable due to degraded frequency response. In the aforesaid application, there is also disclosed a system which assures good sound quality reproduction in respect to stereo signals with lesser degree of correlation. However, reproduction of signals with higher degree of correlation was found to be unacceptable in terms of sound quality.